GEC BT302: poor picture quality.

[FERNSEH]

The intention is to use this GEC TV as part of a display vintage domestic electronics.
But the picture quality will have to be improved, at it moment it's rubbish.
We've got to dispel all those popular misconceptions about the picture quality of these old sets.
Also the set has that "picture ballooning" fault.

DFWB.

[MALC SCOTT]

Hi David, looks like the phantom twiddler has been there! Malc

[FERNSEH]

Hi Malc,
Lot's of those crunched hexagonal cores in the IF transformers. I'll replace them with ordinary screwdriver slot cores.

DFWB.

[FERNSEH]

So far the re-alignment of the IF transformers is not going very well. The cores and former of the final sound IF transformer was so badly damaged that a replacement had to be made.

DFWB.

[FERNSEH]

The alignment was way out. The sound IF was found to be 39Mhz, should be 38.15Mhz and the vision was peaking at 34Mhz. 34.65Mhz is the correct IF and should be 6db down because of the vestigial sideband characteristic of the transmitted signal.
After a removing many crunched tuning slugs and retuning the vision IF transformers to the correct frequencies a much better picture has been secured.
More faults to sort out: That ballooning picture problem which seems to manifest itself in these sets. Also the contrast control doesn't work.

DFWB.

[mark pirate]

Well done on sorting the alignment David, certainly a real pain after some idiot has wrecked the cores
No doubt you will end up with a decent set, the BT302 is a very capable set.

Mark

[FERNSEH]

Hi Mark,
I've always liked these sets, right from the start when I used repair lots of them in the sixties. Customers liked the these slim GEC sets also.
A replacement U25 has solved the picture ballooning fault. According to the service manual there is a 7.85 ohm resistor in series with the U25 heater, it's a 6.2 ohm component in this set. I'd reckon this resistor could be reduced even more so, say 5.6ohms?
What used to happen to these sets was the Z329/30F5 IF amplifier valve would go low emission and the common EF80 used as a quick replacement instead. Pin compatible except for the fact the inter electrode capacitances are different. So the picture didn't look right, so it was out with the twiddling tools which of course more than often was the incorrect trimming tools so we get the dreaded crunched tuning slug problem.
Back in the sixties we would simply order new transformers and coils from the service department.
The attachment shows the improved picture quality. Unfortunately the camera has captured only one picture field and as a result it looks rather liney.

DFWB.

[mark pirate]

That looks more like it I had a BT302 back in the late 70's, it had a superb picture and was in immaculate condition.
I bought it from a scouts jumble sale for £2! Another set I should have kept...

Mark

[beery]

Hi David,
that's a fantastic picture.
I don't have one of these. I have a similarly super picture on my HMV BRC 981 set (a 16" BRC980) which was my first 405 line set, given to me by my Physics teacher. Gerry Wells helped me overhaul it many years ago.

Cheers
Andy

[Heatercathodeshort]

GEC had similar ideas to the Ekco 3 link chassis, building upon a reliable design as technology improved.
The GEC BT302 has many things in common with the 9" BT2147 of 1949.
The whole series were very reliable and predictable and were very well liked by their owners. I have always thought it a shame that the GEC factory at Coventry ceased television production in favour of the Sobell plant at Slough when the two companies joined in 1962. Sobell were an excellent company but I think the Coventry factory had the edge on quality production. John.

[FERNSEH]

Circuit diagram of the tuner unit.

According to the service manual L7 the output coil should not be disturbed but if does become necessary to readjust it the procedure is as follows:

Reconnect the co-ax cable to the IF sub deck. Connect the signal generator set to 34.65Mc/s to the test point in the grid circuit of V2A, and adjust L7 for maximum vision response. further slight adjustment of L7 may be necessary to bring the 2.8Mc/s response up overall.

Well, if this procedure is carried out the picture quality is spoilt completely.
The output coil L7 is extremely peaky and only way to secure a perfect picture is to detune it well away and below 34.65Mc/s.
Then the gain of the receiver is degraded. It might necessary to introduce a damping resistor across L7 to flatten it's response.

DFWB.

[FERNSEH]

A damping resistor has been added across the tuner IF output coil L7.
I kicked off with 6.8Kohms and later on a 12K resistor was fitted instead. it's a compromise between gain versus picture quality.
The response curve of L7 has been flattened and that has got rid those ringing effects, nice clean pictures now.

Picture ballooning fault: that was solved just by replacing the U25 EHT rectifier. However, it's worth noting a degree of beam limiting action is done by the resistor and capacitor combination which feeds the cathode of the CRT, R67 and C151. Also, some BT302 sets have a 1Mohm resistor connected between the HT line and the cathode of the CRT.

DFWB.

[Silviefisch]

Thanks for all your hard work David, can't wait to see it in action again!

[FERNSEH]

Behold the wonderfulness of the GEC BT302.

A further improvement in gain and definition was secured by raising the value of the damping resistor of tuner IF coil L7 to 18Kohms.
The combined volume and contrast control has been repaired so now the contrast control works. Replacing these combi control assemblies was one of the dread jobs of past TV servicing times.
I knew that this was always was a good set, strange how it had deteriorated from many years of lack of use.

DFWB.

[Heatercathodeshort]

That is interesting David. I used to fit a 22K across the IF coil to flatten the response a bit and give a less critical adjustment of the fine tuner.
Many examples of this model tuned in a slightly different manner to the norm requiring a s harp null in the fine tuning to prevent sound on vision.
I was often driven to despair to see the destruction of IF transformers and fireball tuner oscillator cores by what can only be described as idiots. The 1960-70 period appears to be the height of these activities! Philco 1000/1019 [Every single example] Sobell TPS173 and Thorn 850 were the unfortunate subjects of this evil practice. Mind you it did give us the skill to align these receivers correctly with the aid of the Advance E2 and a little patience.
Your example looks a cracker! Regards, John.

[Nuvistor]

Another great job, some 405 TV's were capable of excellent results.

I must have been lucky with phantom twiddlers, hardly saw any.

Frank

[FERNSEH]

Hi Frank,
used to come across twiddled sets all the, as John says the 60s was the worst time in TV servicing history for this sort of thing.
What the bodgers couldn't understand was that overloading by strong signals caused vision on sound and sound on vision effects.
The GEC BT302 was also often a victim of the twiddlers.
The signal generator was kept busy realigning these unfortunate sets.

After the BT302 the next series of GEC sets were supplied with a modified IF panel. The Philips beehive trimmers employed in the sound take-off and sound trap were replaced by conventional coils which are adjusted by iron dust cores. The DH77/6AT6 and N369/30P12 sound amplifier valves were replaced by a single PCL82.
So ended the design legacy dating back to 1951 starting with the model BT2147.

DFWB.

[FERNSEH]

I reckon it's OK to sign off this job. The attachments show the position of the damping resistor. Note the Hunts capacitor. I'm not taking chances with the reliability of this set so that component has been replaced with a Mullard 1000pF polyester mustard capacitor.
The combined contrast and volume looks an awful thing but it is working fine after repairing the rotation limit stops in the contrast pot.

DFWB.